There are various methods available for preventing performance degradation due to inter-cell interference in an environment where a single network service provider operates multiple cells by using multiple access points (APs).
Methods for managing inter-cell interference can be divided largely into two groups: methods dividing neighboring cells and frequency bands so that separate frequency bands having no interference upon each other are allocated to the respective cells and those methods using the same frequency bands but reducing interference on the neighboring cells by reducing power consumed in a cell affecting large interference on its neighboring cells through appropriate power control.
It is well known that in case a method for controlling inter-cell interference through power control employs only one channel (or carrier), an optimal form of power control controls only the minimum and maximum power rather than increase or decrease the power in an appropriate manner. Such kind of control is usually called bang-bang control, and the on-off state of each wireless infrastructure can be defined as a pattern.
FIG. 1 illustrates one form of pattern control for controlling inter-cell interference in the case of two cells.
With reference to FIG. 1, in the case of two wireless infrastructures, a total of three patterns are possible, consisting of ‘pattern 1’ employing both of the two infrastructure, ‘pattern 2’ employing only first infrastructure, and ‘pattern 3’ employing only second infrastructure.
In the case of ‘pattern 1’, two wireless infrastructures are activated at the same time and inter-cell interference is caused; however, in the case of ‘pattern 2’ or ‘pattern 3’, only one infrastructure is deactivated, thus interference in the remaining cells can have a channel state. One disadvantage should be noted that the number of data that can be sent at the same time is reduced from two to one for ‘pattern 2’ and ‘pattern 3’.
In the conventional method, one service provider operates multiple wireless infrastructures and applies various means to avoid performance degradation due to interference among cells constructed by each wireless infrastructure. In this case, the inter-cell interference control is implemented through determining activation and deactivation of each wireless infrastructure at each time slot, namely through determining a pattern for each time slot.
In what follows, briefly introduced will be a method for controlling interference among cells constituting each wireless infrastructure according to the pattern method.
First, the following terms are defined for the convenience of description.
N: a set of wireless infrastructures or cells, may also means the number of the set of wireless infrastructures or cells.
S: a set of users, may also means the number of the set of users.
Sn: a set of users serviced in a wireless infrastructure n, may also means the number of the set of users serviced in a wireless infrastructure n.
P: a set of patterns, may also means the number of the set of patterns.
NP: a set of wireless infrastructures activated according to pattern P.
RS(t): average transmission rate obtained by a user S up to time t.
RS,P(t): instantaneous transmission rate allowed for a user S at time t in the case of pattern P.
α: parameter meant for adjusting network efficiency and fairness among users.
β: constant required at the time of updating average transmission rate.
Algorithm for pattern selection and scheduling of users (terminals) within an AP region
1. The AP measures an instantaneous transmission rate RS,P(t) of the user for each pattern at the start of each time slot. And the AP informs a central node (CN) of the measured instantaneous transmission rate RS,P(t) or transmits RS,P(t) to the central node. It is common that the central node and AP are connected to each other wired but can also be connected to each other wirelessly.
2. The central node determines an optimal pattern p* through the following equation 1 among the patterns indicating on-off states of individual APs based on the instantaneous transmission rate of the user at each time slot.
                              p          *                =                  argmax          ⁢                                    ∑                              n                ∈                                  N                  p                                                      ⁢                                                  ⁢                          [                                                max                                      s                    ∈                                          S                      n                                                                      ⁢                                                                            r                                              s                        ,                        p                                                              ⁡                                          (                      t                      )                                                                                                  [                                                                        R                          s                                                ⁡                                                  (                                                      t                            -                            1                                                    )                                                                    ]                                        α                                                              ]                                                          [                  Eq          .                                          ⁢          1                ]            
3. In the wireless infrastructure n∈Np activated according to pattern p*(t) determined through step 2, the user (or terminal) to be serviced is determined by the following equation 2.
                              S          n          *                =                  arg          ⁢                                    max                              s                ∈                                  S                  n                                                      ⁢                                                            r                                      s                    ,                    p                                                  ⁡                                  (                  t                  )                                                                              [                                                            R                      s                                        ⁡                                          (                                              t                        -                        1                                            )                                                        ]                                α                                                                        [                  Eq          ⁢                                          .                                          ⁢          2                ]            
4. Based on the result of step 3, average transmission rate Rs(t) is updated according to the following equation 3.
                                          R            s                    ⁡                      (                          t              +              1                        )                          =                  {                                                                                                                                                                  (                                                      1                            -                            β                                                    )                                                ⁢                                                                              R                            s                                                    ⁡                                                      (                            t                            )                                                                                              +                                              β                        ⁢                                                                                                  ⁢                                                                              R                            s                                                    ⁡                                                      (                            t                            )                                                                                                                                                                                                  if                        ⁢                                                                                                  ⁢                        s                                            =                                                                        s                          n                          *                                                ⁡                                                  (                          t                          )                                                                                                                                                                                                        (                                                  1                          -                          β                                                )                                            ⁢                                                                        R                          s                                                ⁡                                                  (                          t                          )                                                                                                                                                                        if                        ⁢                                                                                                  ⁢                        s                                            =                                                                        s                          n                          *                                                ⁡                                                  (                          t                          )                                                                                                                                ⁢                                                          ⁢                                                          ⁢              for              ⁢                                                          ⁢                                                ∀                  s                                ⁢                                  ∈                                      S                    n                                                                        ,                          ∀                              n                ∈                N                                                                        [                  Eq          ⁢                                          .                                          ⁢          3                ]            
5. After t=t+1, step 1 is resumed.
As retention of Wi-Fi networks is regarded to be strong competitiveness, APs for Wi-Fi networks are being installed competitively by wireless communication service providers such as SKT, KT, and LGT. Installation of APs is also getting much attention by wireless communication service providers since the existing cellular infrastructure alone cannot accommodate explosion of mobile traffic along with widespread of smart phones. Accordingly, many of mobile communication service providers have started installing a large number of APs, where APs are installed being overlapped in such areas as important hot spots.
Therefore, in those areas where radio coverage of APs overlaps with each other, performance degradation due to inter-cell interference in WLAN has come to the fore. In view of service providers, costs incurred due to thoughtless deployment of APs can be described by the situation where radio resources are scarcely available due to interference from neighboring APs in spite of costly installation of APs. In particular, it can be a problem if newly introduced APs installed by other service providers cause the aforementioned situation.